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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

290

button wrinkled and crystalline, but less than 2% of platinum does not
influence the results since, in presence of silver and gold, it then dissolves
in the nitric acid. If the platinum is present in higher proportions (up
to 10-15%), the cupellation is carried out at a higher temperature in presence
of twice as much lead as is indicated in the table ; after the parting the
comets are weighed, again subjected to inquartation with silver and 20
per 1000 of copper and again parted with acid of D 1-3, these operations
being repeated until the cornets are of constant weight.

Palladium has no injurious effect, since it dissolves completely in nitric
acid.

Indium causes the formation of black spots on the button, these remain-
ing even after parting. If the gold is dissolved in aqua regia, the indium
remains undissolved and may be collected and weighed.

ALLOYS   OF   GOLD,   SILVER  AND   COPPER

In the assay of these alloys, three cases are distinguished:

(1)  Rich alloys, in which the proportion of gold to silver is higher than
1:3.

(2)  Medium alloys, with a proportion of about 1:3.

(3)  Poor alloys, with a proportion less than I : 3.    The assay of these
alloys includes two operations:

(1)  Cupellation, which gives the gold and silver together.

(2)  Parting, which gives the gold alone.

Preliminary test. In presence of silver the touchstone assay does not
always give reliable results (the presence of silver is easily detected by the
formation of a slight, white precipitate on the streak when treated with
the acid), so that a preUminary assay by cupellation is advisable. This is
made with 0-250 gram of the sample and 4-8 grams of lead, according to
the supposed richness in copper, the button of gold and silver being weighed ;
the weight of the sample, less that of the button, gives approximately that
of the copper. The button is then rolled, the strip treated with nitric acid
and the remaining gold weighed ; the proportion of silver is then found by
difference.

I. RICH ALLOYS, (a) Determination of the gold. This is carried out on
two separate portions of 0-5 gram under the conditions given for the deter-
mination of gold in gold-copper alloys. The inquartation silver must be
diminished in amount by the approximate silver content indicated by the
preliminary assay (see p. 287). The amount of lead required is based on
the total fineness and on the ratio between the gold and silver (see later:
Medium Alloys).

(b) Determination of the silver. 0-5 gram of the alloy is cupelled with
the quantity of lead used for the determination of the gold, the resulting
button representing gold and silver together; the latter is thus found by
difference.

If the alloy consists of gold and silver alone, 20-thousandths of copper
must be added to the test. If the gold fineness is less than 800, the third
treatment with acid is omitted.the same conditions. In assaying commercial fine gold Riche advises the